real-time graphics architecturegraphics.stanford.edu/.../lectures/opengl/opengl.pdf ·...

7/9/2007

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Real-Time Graphics Architecture

Lecture 16: The Design of OpenGL

p

Kurt Akeley

CS448 Lecture 16 Kurt Akeley, Pat Hanrahan, Spring 2007

Kurt Akeley

Pat Hanrahanhttp://graphics.stanford.edu/cs448-07-spring/

Early SGI history (dates are approximate)

1975 1980 1985 1990


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Historical context (circa 1990)

SGI is the dominant workstation 3-D graphics player

High-end multiprocessor: PowerSeries GTX, VGXDesktop: Personal Iris, Indigo

Workstation 3-D APIs are proprietary, not interoperable

IRIS GL (SGI)Starbase (HP)

…IRIS GL is a mess

Evolved (accreted) by SGI from 1982 onPoorly specified with inconsistent implementations


Poorly specified with inconsistent implementationsIBM licensing deal

GTX/Personal IRIS development

PC is becoming an interesting platform

Intel 486 included on-board FPU and cache memoryMicrosoft exploring 3-D collaboration with SGI

PHIGS/PEX history* (dates are approximate)

1975 1980 1985 1990


* Summarized from http://www.jch.com/~jch/vrml/PEXTimes.html

** Programmers Hierarchical Interactive Graphics System

*** Convex, DEC, E&S, HP, IBM, Kubota, ShoGraphics, Stardent, Sun, Tek

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PEX (unofficially the PHIGS Extension to X)

PEX


The PEX threat (circa 1990)

PEX considered sub-optimal by SGI

Retained scene graph* (immediate mode desired)

X protocol extension (API desired)

Differed from IRIS GL

Designed by committee

NIH (maybe, we had ignored it)


But PEX will soon be very real …

* Actually, while PHIGS had no immediate mode, PEX had been extended by this time to include one

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The (controversial) choice

Remain proprietary

Evolve IRIS GL

Drive standards

PEX

Our own initiativeInternally called GL5

Became OpenGL


Talk outline

Goals

Design decisions

Successes and failures

Moving forward


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Goals


Goals for OpenGL

Industry-wide acceptance (beat PEX!)

Consistent implementations

Innovative implementations

Innovative and differentiated applications

Long life

High quality


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Non-goals

Make graphics programming easy

OpenGL is a power tool

You might lose a finger (i.e., see a black screen)

Integrate digital media and 3D graphics

This is a much larger task

See discussion of Khronos later in the talk


Goal: Industry-wide acceptance

Avoid compromising performance

Allow explicit application trade-offs (modes)

Get it right the first time

Make conservative changes from IRIS GL

Collect lots of input during design

Do implementation during design

Create an open standard


Create an open standard

Licensing program

Architecture Review Board (OpenGL ARB) to control future evolution of specification

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Goal: Industry-wide acceptance (cont.)

Achieve compatibility with multiple …

Operating systems (Windows, IRIX, HPUX, AIX, …)Ultimately Linux, Mac OS, OS/2, Be, …)Ultimately Linux, Mac OS, OS/2, Be, …)

Window systems (Windows UI, X Window System, …)Frame buffer is not part of OpenGL state

Programming languages (C, FORTRAN, Java, …)No pointers, structures, function overloading

No 2-D arrays at interface (row-major / column-major)

T t Mi ft t


Target Microsoft acceptance

Omit 2D-only functions, window interface, font supportAllow full application compliance with Windows standards

Issue: no driver interface specified

Goal: Industry-wide acceptance (cont.)

Marginalize PEX

API, not protocol, is the right interface

But client-server mattered tooDefined GLX protocol

Included server-side display list storage

Carefully specified client-side state

Match current hardware capabilities


Get input from other IHVs

Meet current application needs

Get input from ISVs

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Goal: Consistent implementations

Tightly written specification

Conformance tests and required verification

Complete implementationsComplete implementations

No sub-setting of 1.n specification

Minimum resource requirements specified

Required runtime error semantics (small performance cost)

Check and report

No other side effects


Incentives to share extensions

Balance desires for consistency and for innovation

Result: portable applications

Goal: Innovative implementations

Specification not too tight

Not pixel-exact

Smooth antialiasing loosely definedSmooth antialiasing loosely defined

Abstraction sometimes tilted in implementor’s favor

Texture images and filters are paired

Evaluated attributes do not become “current”

Extensibility


Key to IHV innovation

Requires IHV control of entire driverObvious for hardware systems companies

Not at all obvious for Microsoft

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Extensibility

Registry is maintained on www.opengl.org

Over 340 extensions so far

Names, token values, GLX protocol, …

Careful extension documentation

Suffix/prefix rulesClearly identify all non-core commands and tokens

Extension numbers


Must account for all lower-numbered extensions

OpenGL is more than the sum of its parts

Extension specification template

Extension template

Name

Name strings

Version

Dependencies (details)

Errors

New state (table format)Version

Number

Dependencies (list)

Issues

Overview

New procedures and functions

New state (table format)

Get Value

Get Command

Type

Initial Value

Attribute Set


New tokens

Additions to chapter [2,3,4,5,6]

Additions to GLX specification

GLX protocol

New implementation-dependent state (table format)

Usage examples

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Extension syntax rules

Abbreviations (required, specifically not allowed)

Compound words (required)

Naming rules

General (e.g., “depth” rather than “z”)

Procedures (e.g., verb-noun or adjective-noun)

Defined constants (all caps, underbars)

Parameters ([0],1,2, <target>, <params>)


Parameters ([0],1,2, <target>, <params>)

Extensions (prefixes, suffixes)

Parameter order and typing rules

Suffix codes

Extension categories

Proprietary

Use corporate prefix/suffix (e.g. NV)

EXTEXT

Use EXT prefix/suffix

Must be implemented by at least two licensees

ARB

Use ARB prefix/suffix


Specification controlled by the ARB

1.n

No prefix/suffix

Specification controlled by the ARB

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Goal: Innovative applications

Design qualities that encourage innovative useImperative (procedural) interface

In X-speak, “mechanism, not policy”

Orthogonal mechanismsData recirculation

Similarity of pixels and texelsMachine shop analogy

Sufficiency of capability (e.g., stencil, blending)Intuitive usability (not ease of use)Invariant operation


Invariant operationTaken together, these design qualities allow

“programmable” operation, in the multi-pass senseUsages that were not anticipated by the designers

But application developers will not innovate if implementations are not consistent!

Goal: Long life

Extensibility

Enabled

Directed (process in place)

Strong foundation

Precise specificationAnd culture that values it

Anticipation of important trends


p pTexture mapping

Integration of image processing and 3-D graphics

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Goal: High quality

Aim for beauty, not just sufficiency or correctness

SGI culture supportive of this

Avoid design-by-committeeAvoid design by committee

Build a horse, not a camel

Correct well known IRIS GL deficiencies

No command prefixes

Can’t be used by libraries (incomplete save/restore)

Error reporting via printf() !


Provide documentation

Specification

Man pages

Programming guide

Design Decisions


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Unit-square pixels and (matching) texels

Square

Exact tiling of window area

Matches window system ownership

(1,1)

y pmodel

No window compositingWIDS (later: CIDs and DIDs)

Correct frustum arithmeticConsider single-pixel frustum

Correct texture wrapping

N d i l i

(0,0)


Not centered at integral units

Non-negative in window

Obvious viewport arithmetic

Matching pixels and texels

Obviously important (in retrospect!)

(0.5,0.5)

No direct render to texture

Pixel and texel coordinates are equivalent

But pixels and texels are still distinguished

Allows implementation with separate memories

Supports OpenGL state modelTextures are OpenGL state

Frame buffer contents are not

– Allows mutiple renderers to share frame buffer


An unnecessary orthogonality?

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No return values (except from queries)

Status return values are a common library feature

But they have horrible performance implications

Expose pipeline latency

Expose client-server latency

OpenGL error system carefully engineered to be

Asynchronous

Distributed


Distributed

void glBegin(GL_POLYGON);

GLstatus glBegin(GL_POLYGON);

Evaluated attributes do not become current

“Correct” semantics

Force separate hardware for evaluation and vertex Evaluationfor evaluation and vertex ops

Defeat “unified” implementation

Compromise allows practical implementation

Evaluation

“Smear” currentvalues


Vertex

Operations

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More design decisions

Object IDs (not handles)

No return value ☺

Scriptable

Sifdv interface

For application convenience and efficiency

Discourage incremental matrix arithmetic

No pre-multiply


No pre multiply

Stack provided for undo

Application-specified clipping done in eye coordinates

Allows use of singular projection matrixes

Successes and Failures


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Success with respect to goals

Industry-wide acceptance

Great for Unix, Linux, Apple, embedded systems

Good as can be expected with Microsoft ☺

Poor for game consoles

Consistent implementations

Good for core features

Marginal for new features


Marginal for new features

Innovative implementations

Industry and academic standard

Interesting work-arounds for constraints

Success with respect to goals (cont.)

Innovative and differentiated applications

Very successful

Long life

Fifteen years and counting

High quality

Generally respected

But implementations on Windows suffer as Direct


But implementations on Windows suffer as Direct 3D becomes more important

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Technical successes

Imperative interface

“Every graphics systems aspires to have an imperative interface”p

Generalized texture capability

All primitives (with pixel texture extension)

Various dimensions of texture

Texel and pixel conformance

Incremental in-place modification


Incremental in place modification

Explicit format, filter specification, MIP level, …

Texture matrix

Formalization of fragment

Mistakes

Business

Taking our eye off Microsoft

TechnicalTechnical

Display lists as encapsulation for “objects”Forced to introduce texture objects in version 1.1

Overly-abstract storage managementProxy-priority middle ground is not tenable

Few “fast path” clues in API


Too much faith in pure mechanism

E.g., DL objects, window position, 2-D transform, in-place frame buffer to texture memory transfer, …

Persistent parameter state between Begin/EndBut would have broken IRIS GL programs badly

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Mistakes (cont.)

Probably should have been omitted

Texture borders! (but not texture-border color)

Edge flagsEdge flags

Smooth polygon antialiasing

Color index features (lighting, antialiasing, …)

Needlessly complex or wrong

ColorMaterial()

Normal transformation


Texture wrap semantics

Bit fields (cannot support extensions)

Viewport should accept doubles, not ints

Presumption of “view direction” (lighting, fog)

“While the quality of tangible goods has increased dramatically over the last 30 years the quality of software artifacts

Did OpenGL help SGI?

years the quality of software artifacts has been steadily declining.”

— Alex StepanovIn Industrializing Software Development,Zhejiang University 2004

CS448 Lecture 16 Kurt Akeley, Pat Hanrahan, Spring 2007 36

Zhejiang University, 2004

Software quality and corporate success may be inversely correlated

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Lessons

It is worthwhile to specify carefully

Don’t complete the implementation before starting on the specification!starting on the specification!

It is critical to guide system implementers and application developers to a common understanding of how to support and achieve high performance.

People make and evolve standards

Invest in developing a culture


p g

Keep competitive issues out of a specification’s controlling body.

Moving Forward


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Future of OpenGL (from 2001 talk)

On the rebound

Low period in the late 90s

Provides alternative to DirectX/Microsoft API control

Apple commitment is a positive sign

Programmability is an exciting development

Fits well in OpenGL structure

OpenGL may truly be a “library” some day


OpenGL may truly be a library some day

Lots of 2.0 activity, ARB still functioning

Tenth birthday this summer (at SIGGRAPH)

Khronos - Open Media Standards

Open International MembershipAny company is welcome

Funded by membership dues - $6K / year

Open StandardsPublicly available on web-site

To be used by any company in any productsu ded by e be s p dues $6 / yea y y p y y p

Open Standards for Media Authoring and Acceleration

Creating Market OpportunitiesBringing together industry’s leading media technology experts


Bringing together industry s leading media technology expertsInvesting hundreds of man years to create state-of-the-art media API standards

Making those standards available to the industry – ROYALTY-FREE

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Over 100 companies creating media authoring and

acceleration standards


Khronos Standards Ecosystem

Cross platform desktop 3D

Cross-platform graphics authoring/acceleration ecosystemSafety Critical 2D/3D

All open standards for native 3D graphics acceleration and authoring are now being

developed in Khronos. Tremendous opportunity for collaboration and synergy

Dynamic Media Authoring

Dynamic Media Authoring Standards

Cross platform desktop 3D

glFX3D Authoring Effects

Framework


“DirectX-like” set of native APIsIncludes mixed media acceleration and OS portability APIs

Embedded Media Acceleration APIsVector 2D Streaming Media Enhanced Audio

Composition Working GroupHardware acceleration for window systems

2D/3D

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Khronos Participation Model

PromotersBoard decides strategy – approves working groups, controls budget,

ratifies specifications Contributors

Any company can join Khronos to participate in any number of working

groups to produce specifications.$6,000 annual membership dues

Conformance Tests

RatifiedSpecifications

Conformance Tests and Conformance Test Process.Typically $10K per API fee

Openly and publicly distributed – free of charge,

royalty free

ratifies specifications. $20,000 annual membership dues

SDKs

A Working Group for each API standard –

one company one vote


AdoptersImplementers

Anyone can download specifications and SDKs and implement royalty-free products

Conforming products can use API

trademark and logo

Free libraries, utilities, examples

Khronos Conformance Process

Non-MembersKhronos

Contributor or Promoter

Become Adopter Member ($2,500

annual fee)

Sign Adopters Agreement to access Test Source and

Process(typically $10K

per API)

Port Test Source to

product and generate test

results

Upload test results to

Khronos private web-site for peer review by other

Adopters

Successful Review means

products can use Khronostrademarks

Members


2D/3D

Vector 2D

Streaming MediaAlready released

COLLADA, OpenKODE coming soon

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SGI / Pixar collaboration

“It seems to me that we are trying to merge troff and Scribe into a single hodge-podge. I don't think that we'll get a good result if we continue.”

“It seems that a key to a high-performance interface is choosing the right boundary between what and how. … Even the Pixar language has this boundary in it. It still does not support a description like 'lighted like a Sunday afternoon in September'.”

“I am in touch with Pat Hanrahan. He will send me an


updated copy of a document outline today. I am to be working on what I discuss in this note. What next?”

— Kurt Akeley (to Forest Baskett)

Fahrenheit

SGI promoting and supporting PC OpenGL

Microsoft asserting control of their own platform

Fahrenheit is the negotiated settlement

Results:

Broad reach of agreement failedNo new low-level standard, little at scene graph level

OpenGL still exists on all Windows platforms


p p

SGI learned a lot about Microsoft’s business

Microsoft learned a lot about graphicsSeamus’s presentation at Graphics Hardware 2001

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Direct3D

For years Direct3D has patterned itself after OpenGL

Immediate mode

Features such as stencil buffer

There are surprising examples in Direct3D 10

Geometry shaders (anticipated by evaluators)

Unified shaders (anticipated by evaluators)

Stream out (anticipated by feedback)


Stream out (anticipated by feedback)

Even GPU programmability is not really new

Recall earlier lecture

But also …

From the Wikipedia article on PHIGS:

The rise of OpenGL and the decline of PHIGSOpenGL, unlike PHIGS, is an immediate-mode rendering system with no "state";

once an object is sent to a view to be rendered it essentially disappears. Changes to the model have to be re-sent into the system and re-rendered Changes to the model have to be re-sent into the system and re-rendered, dramatically increasing programmer workload. For simple projects, PHIGS was considerably easier to use and work with.

However, OpenGL's "low-level" API allowed the programmer to make dramatic improvements in rendering performance by first examining the data on the CPU-side before trying to send it over the bus to the graphics engine. For instance, the programmer could "cull" the objects by examining which objects were actually visible in the scene, and sending only those objects that would actually end up on the screen. This was kept private in PHIGS, making it much more difficult to tune performance.


Given the low performance systems of the era and the need for high-performance rendering, OpenGL was generally considered to be much more "powerful" for 3D programming. PHIGS fell into disuse. Version 6.0 of the PEX protocol was designed to support other 3D programming models as well, but did not regain popularity. PEX was mostly removed from XFree86 4.2.x (2002) and finally removed from the X Window System altogether in X11R6.7.0 (April 2004).

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OpenGL specifies an architecture

OpenGL is an API-based architecture specification

Low-level imperative interface (like an ISA)

Tight specification (like an ISA)

Orthogonal operation (like an ISA)

This is what mattered most

Allowed hardware innovation


Allowed hardware innovation

Allowed application innovation

Allowed programmability

Deferred need for “shaders”, then allowed them

“In terms of stifling innovation, good ideas are far more dangerous than badideas are far more dangerous than bad ones. They take hold, assume momentum, and therefore result in inertia. Consequently, they are hard to displace, even when they are well past their prime.”

CS448 Lecture 16 Kurt Akeley, Pat Hanrahan, Spring 2007 50

— Bill BuxtonIn Sketching User Experience,Morgan Kaufmann, 2007

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To learn more

www.opengl.org

www.khronos.org


Real-Time Graphics Architecture

Lecture 16: The Design of OpenGL

p

Kurt Akeley


Kurt Akeley

Pat Hanrahanhttp://graphics.stanford.edu/cs448-07-spring/

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